The present invention relates to electric power assisted steering systems (EAS) and is concerned in particular with a method and apparatus to be incorporated in a control system for an electrically powered steering system for compensating for inertia within the system generated by the electric assist motor.
Typically, an EAS system comprises a driver""s steering wheel or handle through which the driver inputs a steering demand. The steering demand produces a torque input which has both magnitude and directional sense. The steering wheel is connected to a steering column/shaft which carries within a portion of the shaft a torque sensor for measuring the driver""s steering demand. Preferably the sensor, either alone or in combination with additional sensors, provides signals corresponding to the steering wheel position, which can be further used to determine steering speed and thus steering acceleration. All of these signals are measurements corresponding to the driver""s steering input or steering demand.
The driver""s steering demand is processed in accordance with a basic steering torque servo control which provides an output signal, dependent upon the driver""s demand, to an electric assist motor, which adds in steering assist torque into the steering shaft, downstream of the torque sensor. The electric assist motor is typically connected to the steering shaft by some form of gearbox, the ratio of which is set by determining the range of assistance torque and system response required for a particular system installation. Finally, the steering shaft is operatively connected to the steerable road wheels by a rack and pinion steering assembly, which could optionally carry the electric assist motor instead of the steering shaft depending upon the particular installation constraints.
In operation, the system has to cope with a wide variety of driver""s steering inputs and other external demands as imposed on the system during normal running, such as speed dependent self centring torques developed at the road wheels, or torsional fluctuations developed as the road wheels pass over a changing road surface. There is a need to compensate for each of these additional external system inputs if the whole steering system is to remain stable under all operating conditions.
One particular disturbance, developed within the electromechanical steering system itself, for which compensation needs to be provided, is the torsional drag induced within the steering system as a result of inertia predominantly within the electric assist motor.
It is one object of the present invention to provide a method and apparatus to compensate for torsional drag induced within the steering system as a result of reflected system inertia.
It is known within EAS systems of the present kind to add in a term to the assistance torque calculation that compensates for the above described reflected system inertia. As described for example in GB 2175551 and U.S. Pat. No. 5,740,040, this is usually done by providing a value which is proportional to the steering acceleration as derived by differentiating the steering column velocity. It has been found however that the base steering column velocity signal as measured at the steering column carries not only the driver""s steering demand velocity but also other shifting disturbances as described above which, when differentiated, induce acceleration terms into the inertia compensating calculation which has the potential to make the system unstable. Typically, the driver would be able to input a steering demand which could develop a measurable change within the system at a rate of up to 5 Hz, as could be demanded during a xe2x80x98panic steering manoeuvrexe2x80x99 to avoid an obstacle. Noise, developed from external background sources, such as running over a rough road, would typically be at higher frequencies such as in the region of 10 to 20 Hz or higher. Another object of the present invention is to compensate only for those changes within the system that can be directly attributed to the driver""s steering demand, i.e. below a frequency response which could be reasonably achieved by the driver.
In accordance with the present invention there is provided an electric power assisted steering system of the kind in which an electric motor is operatively connected to a steering assembly through a gearbox to apply an assistance torque to the steering assembly and in which the steering assembly includes a steering column coupled at one end to a driver""s handwheel and at the other end to steerable wheels, a means being provided for generating a signal representative of the velocity of rotation of the steering column, wherein the system includes a high pass filter means which is adapted to generate, for frequencies below a prescribed threshold, a signal at least approximately proportional to steering column acceleration and representative of the inertia of the rotor of said electric motor, the latter signal being arranged to be used to compensate for said motor rotor inertia in an overall assistance torque demand as developed by the system.
In one preferred embodiment, the steering column acceleration signal is applied to a gain control means to provide a torque output signal which is factored to take account of the ratio of the gear box employed in the system between the motor and the steering column or steering linkage.
Preferably, the factored torque signal is passed through a limit means which imposes a limit on the magnitude of the developed torque signal to produce a capped assistance torque demand signal. The capped assistance torque demand signal can then be added to a basic servo assistance torque demand provided by the system whereby to provide a compensated overall assistance torque demand for the system.